Field of Disclosure
The present disclosure relates generally to a phase locked loop (PLL) and specifically to spur suppression in a free running oscillator for the cellular phone.
Related Art
Cellular phones have evolved from large devices that were only capable of analog voice communications to comparatively smaller devices that are capable of digital voice communications and digital data communications, such as Short Message Service (SMS) for text messaging, email, packet switching for access to the Internet, gaming, Bluetooth, and Multimedia Messaging Service (MMS) to provide some examples. In addition to these capabilities, the cellular phones of today have additional non-communication related capabilities, such as a camera with video recording, an MPEG-1 Audio Layer 3 (MP3) player, and software applications such as a calendar and a phone book, to provide some examples. Even in light of these capabilities, manufacturers of cellular phones are placing even more capabilities into cellular phones and making these more powerful cellular phones smaller.
At the heart of each cellular phone lies a phase locked-loop (PLL) to generate various electronic signals within the cellular phone. For example, the PLL is responsible for providing various clocking signals within the cellular phone. As another example, the PLL is responsible for providing various electronic signals for transmission and/or reception of data. Some cellular phones also include a free running oscillator, such as a crystal for example, that is external to the PLL. The free running oscillator provides an oscillation signal having an oscillation frequency to the input of the PLL. The oscillation frequency is not fixed, rather, the oscillation frequency may vary or drift over various conditions, such as temperature or time to provide some examples.
Conventionally, the oscillation signal, or an integer or fractional multiple of, may be used as a reference signal by the PLL. The PLL adjusts its output such that its output is related, in either frequency and/or phase, to the oscillation signal. However, as the oscillation signal provided drifts, so does the output of the PLL. This drifting may generate spurs that degrade the performance of the cellular phone.
The present disclosure will now be described with reference to the accompanying drawings. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the reference number.